Thermal transfer printing device for transferring a printing image onto a recording medium

ABSTRACT

A printing image is transferred onto a first fabric tape (1), which can be wetted with a liquid (19), at a printing image transfer point (3) as a result of the partial evaporation of the included liquid (19). At a subsequent transfer printing point (12), the first fabric tape (1) is located between a heating device (10) and a second fabric tape (4). Printing ink (11) is included in the second fabric tape (4). This printing ink (11) is expelled from the second fabric tape (4) by the liquid (19), which remained in the first fabric tape (1) and represents the printing image and which evaporates at the transfer printing point (12), and is transferred onto a recording medium (9).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal transfer printing device fortransferring a printing image onto a recording medium in a printer orcopier.

2. Description of the Related Art

U.S. Pat. No. 4,731,619 discloses a thermal printing head of a thermaltransfer printing device. The thermal printing head is arranged at atransfer printing point. At this transfer printing point, a printingimage is transferred onto a recording medium. A printing ink film istransported between the thermal printing head and the recording mediumin a direction of transport of the recording medium. This printing inkfilm has a plurality of small orifices, in which printing ink iscontained. This printing ink is introduced into the orifices of theprinting ink film in an inking station. At the transfer printing point,the printing ink film slides over the thermal printing head and at thesame time is preheated. The thermal printing head has a plurality ofindividually activatable heating elements which serve for the rapidheating of the printing ink in the orifices of the printing ink film.The heating elements are activated by a control unit in accordance withthe printing image to be reproduced. The rapid heating of the printingink causes bubbles to form in the orifices of the printing ink film,with the result that the printing ink is instantly expelled from theorifices in the direction of the recording medium.

By means of the known method, high-viscosity printing inks with a highpigment concentration can be transferred onto the recording medium.Half-tones too can be printed thereby, as a result of which color imagesof photographic quality can also be produced. In the known thermaltransfer printing device, however, considerable requirements are placedon the printing ink. Thus, the printing ink has to be thermally stableup to about 400° C. According to the temperature of the printing ink,the thermal load on the surface of the recording medium also rises. Onlyrecording media which meet these stringent requirements can therefore beused. Since the heating operation is restricted to the relatively smallregion of the printing head, the printing speed obtainable is alsolimited correspondingly.

European Patent Document 0,202,370 discloses a thermal transfer printingdevice having a plurality of thermal printing heads. Each thermalprinting head serves for transferring a printing ink onto a recordingmedium. Each thermal printing head is assigned a cartridge having aprinting ink reservoir, in which the orifices of a printing ink film arefilled with printing ink. The printing ink film is guided over thethermal printing head, where the printing ink is heated and istransferred onto a recording medium as a result of the formation ofbubbles. In multicolor printing, a plurality of transfer printing pointscorresponding to the number of inks are required.

SUMMARY OF THE INVENTION

An object on which the present invention is based is to provide athermal transfer printing device for transferring a printing image ontoa recording medium in a printer or copier, the thermal transfer printingdevice allowing the use of conventional printing inks which do notwithstand high thermal stresses, guaranteeing a low thermal load on therecording medium and permitting a high printing speed.

This and other objects of the present invention are achieved by athermal transfer printing device for transferring a printing image ontoa recording medium in a printer or copier, the device having a firstfabric tape which can be wetted with a liquid, a printing image transferpoint at which the liquid can be partially evaporated out of the firstfabric tape according to the existing printing information, a secondfabric tape onto which printing ink can be transferred in an inkingstation, and a transfer printing point at which the first fabric tapeand the second fabric tape are jointly guided, one on top of the other,over a heating device, the first fabric tape being in touch contact withthe heating device, with the result that the liquid contained in thefirst fabric tape evaporates, and the recording medium can be pressedrelative to the second fabric tape in the direction of the heatingdevice by a pressure element. Developments and embodiments of theinvention are provided by a first fabric tape that is composed of ahigh-grade steel wire fabric. The second tape of the thermal transferprinting device may be a polyester fabric having a fabric thicknessof<100 μm.

Preferably, the heating device extends transversely relative to thedirection of transport of the recording medium and is a heatable roller.The inking station preferably provides, by the selective movement of abacking roll, an ink-carrying roller that can be brought into touchcontact with the second fabric tape. The inking station has at least twoink-carrying rollers are arranged in succession in the direction of runof the second fabric tape.

The thermal transfer printing device wherein the printing image transferpoint has a thermal comb having heating elements activatableindependently of one another, one heating element corresponding to onepixel of the printing image. The dampening unit is arranged at the firstfabric tape and the first fabric tape can thereby be wetted with anontoxic liquid. In particular, the dampening unit contains two rollersrolling one on the other, between which the first fabric tape can betransported through and one of which transfers the liquid onto the firstfabric tape. The thermal transfer printing device uses endless tapes asthe fabric tapes.

By means of the thermal transfer printing device according to theinvention, it becomes possible to use conventional colorants in thermaltransfer printing. The heat effect of the heating device provided at thetransfer printing point is restricted to the liquid contained in thefirst fabric tape or web. This liquid is evaporated and thereby expelsthe printing ink from the second fabric tape or web in the direction ofthe recording medium. The printing ink that strikes the recording mediumis heated only slightly, with the result that the recording medium issubjected to only a slight thermal load. The liquid is evaporated in thefirst fabric tape at a position spaced from the printing location sothat the total energy for producing the image is emitted at two spacedlocations. Since the printing image is produced at two different pointsthe thermal load on the heating device located at the transfer printingpoint is comparatively slight. Additionally, the division of theprinting energy can also be utilized in order to increase the printingspeed.

The fabric tapes have the property of absorbing liquid or printing inkand of partially discharging this material again. The term "fabric tape"therefore also relates to tapes having an irregular structure, such as,for example, a fleece, and all other materials which have thefunctionality mentioned above.

According to one embodiment and development of the invention, thethermal transfer printing device contains an inking station, at which,by the selective movement of a backing roll towards an ink-carryingroller, the ink-carrying roller can be brought into touch contact withthe second fabric tape located between the roller and the backing roll.This guarantees that printing ink is applied only at those points on thesecond fabric tape where this is required. For example, it is necessaryto reload with printing ink only those points on a second fabric tapewhich is an endless tape from which ink was extracted in a precedingtransfer printing operation.

According to a further embodiment of the invention, at leasttwo-ink-carrying rollers are arranged in succession in the runningdirection of the second fabric tape. Consequently, different inks can beapplied to the second fabric tape successively in portions at thedesired points. Multicolor printing is therefore possible with only oneink-carrying fabric tape. Preferably, portions not wetted with printingink are left free between the individual portions of different inking onthe second fabric tape. In the event of a printing interruption, theseink-free regions are then positioned above the heating device of thetransfer printing point.

According to a further development and embodiment of the invention, theprinting image transfer point contains a thermal comb having heatingelements activatable independently of one another, wherein one heatingelement corresponds to one pixel. By means of the thermal comb, theliquid can be partially evaporated out of the first fabric tapeaccording to the existing printing information. The dot size and colordensity, which are to be achieved during the subsequent transferprinting in the transfer printing station, can be determined both byvarying the heating energy on the heating device of the transferprinting station and by varying the heating energy of the heatingelements of the thermal comb. Water is suitable as a nontoxic liquidwith which the first fabric tape can be wetted. During the evaporationof water, no toxic vapors occur which could impair the ambient air ofthe printer or the operating elements inside the printer. The watervapor can be eliminated by means of a suction extraction device.However, other nontoxic liquids which have a corresponding evaporationpoint and do not cause any harmful environmental influences are alsosuitable.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention is explained in more detail below by meansof the drawing.

The FIGURE shows a thermal transfer printing device with a first and asecond fabric tape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE shows a thermal transfer printing device for transferring aprinting image onto a recording medium 9 in a printer or copier. Therecording medium 9 is fed to a transfer printing point 12 in thedirection of transport 14 of the recording medium. At the transferprinting point 12, a printing ink 11 located on a second fabric tape 4is transferred onto the recording medium 9 according to printinginformation located on a first fabric tape 1.

The first fabric tape 1 is designed as endless tape. It is stretchedbetween two rollers 10, 16, the axes of which are parallel. One of therollers 10 is assigned to the transfer printing point 12 and the otherroller 16 is assigned to a dampening unit 2. The first fabric tape 1 isat least as wide as the widest recording medium 9 to be processed. Thelength of the first fabric tape 1 is selected in such a way that thedistance between the two rollers 10 and 16, onto which the first fabrictape 1 is stretched, is so great that a printing image transfer point 3can be arranged in this distance region.

The printing image transfer point 3 contains a heating device 13, bymeans of which a nontoxic liquid 19 contained in the first fabric tape 1can be partially evaporated according to the existing printinginformation. A thermal comb 13, such as is known, for example, from theinitially mentioned U.S. Pat. No. 4,731,619, serves as a heating device13. The thermal comb 13 contains a plurality of heating elements, toeach of which one pixel of the printing information can be assigned.However, a laser or an infrared light source, which are appropriatelycontrollable, could also be used as a heating device 13.

The thermal comb 13 extends over the entire width of the first fabrictape 1. The first fabric tape 1 is guided directly over the thermal comb13. Provided on that side of the first fabric tape 1 facing away fromthe thermal comb 13 is a suction extraction device 18 which sucks awaythe evaporated liquid 19. The liquid 19, which is for example water, canbe condensed in a condensation device (not shown) downstream of thesuction extraction device 18 and fed again to the dampening unit 2.

At the printing image transfer point 3, liquid 19 is evaporated out ofthe first fabric tape 1, designed as a high-grade steel wire fabric witha fabric fineness>200 mesh, at the points at which no application of inkon the recording medium 9 is to take place. After the first fabric tape1 has run through the printing image transfer point 3, therefore, liquid19 is still located in the first fabric web 1 only at the points atwhich an application of ink onto the recording medium 9 is to takeplace.

This liquid 19 remaining in the first fabric tape 1 is evaporated at thetransfer printing point 12 by means of the heating roller 10 serving asa heating device, with the result that the printing image passes ontothe recording medium 9. Consequently, after the run through the transferprinting point 12, there is no longer any liquid 19 located in the firstfabric tape 1. New liquid 19 is transferred into the first fabric tape 1again in the dampening unit 2. The dampening unit 2 has two rollers 16,17 which roll one on the other and between which the first fabric tape 1can be transported through. One of the two rollers 17 dips into theliquid 19 located in the trough of the dampening unit 2. At the sametime, the roller 17 is wetted with liquid 19. The liquid 19 istransferred onto the first fabric tape 1 as a result of the rolling ofthe roller 17 on the first fabric tape 1. The first fabric tape 1, whenit leaves the dampening unit 2, is wetted completely with liquid 19.

The second fabric tape 4, like the first fabric tape 1, is designed asan endless tape. The second fabric tape 1 consists of polyester or ofmetallized polyester with a fabric thickness of <100 μm. The secondfabric tape 4 has at least a width which corresponds to the width of thewidest recording medium 9 to be printed. The second fabric tape 4 isguided over three rollers 10, 20, 21, the axes of which are parallel toone another. In the region of the heating roller 10, the first fabrictape 1 is located between the heating roller 10 and the second fabrictape 4. The direction of movement and speed of the tapes 1, 4 areidentical.

Before it reaches the transfer printing point 12, the second fabric tape4 runs through an inking station 5. In the inking station 5, fourdifferent printing inks 11 are selectively applied to the second fabrictape 4. The inking station 5 can, in this case, be constructed in amodular manner, with the result that any desired number of printing inks11 can be applied to the second fabric tape 4. The application of inktakes place in each case by means of an ink-carrying roller 7 which isarranged on one side of the second fabric tape 4. On the opposite sideof the second fabric tape 4, a backing roll 6 is assigned to theink-carrying roller 7. This backing roll 6 is movable perpendicularly tothe surface of the second fabric tape 4 under the control of a controldevice. If an application of ink is to take place, the backing roll 6 ismoved towards the ink-carrying roller 7. During this movement, thebacking roll 6 strikes the second fabric tape 4 and moves the latterrelative to the ink-carrying roller 7. The second fabric tape 4 and theink-carrying roller 7 thereby come into touch contact, as a result ofwhich printing ink 11 is transferred from the ink-carrying roller 7 ontothe second fabric tape 4. The ink-carrying roller 7 is continuallywetted with printing ink 11 by means of a rolling movement on a supplyroller 22 which dips into an ink supply 11.

The control device ensures that only one backing roll 6 is pressedtowards an ink-carrying roller 7 in each case. Portions of differentinking i.e. different colors of ink, can thereby be produced on thesecond fabric tape 4. Regions without printing ink 11 are providedbetween the individual portions, so that, in the event of a transferprinting interruption, one of these free regions can be guided into theregion of influence of the heating device 10 at the transfer printingpoint 12.

The transfer of the printing image onto the recording medium 9 takesplace at the transfer printing point 12. An evaporation process takesplace at the transfer printing point 12 at those points at which liquid19 is contained in the first fabric tape 1. The recording medium 9 isbrought into touch contact with the second fabric tape 4 by a pressureelement which is designed as a backing roll 8 and which is movablerelative to the heating roller 10 in a direction 15 perpendicular to therecording medium 9. A liquid 19 present at this touch contact point inthe first fabric tape 1 located between the second fabric tape 4 andheating roller 10 is evaporated. The evaporating liquid 19 expels theprinting ink 11 out of the second fabric tape 4, with the result thatthe printing ink is deposited on the recording medium 9.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventor to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of his contribution to the art.

I claim:
 1. A thermal transfer printing device for transferring aprinting image onto a recording medium in a printer or copier,comprising:a first fabric tape, means for wetting said first fabric tapewith a liquid, a printing image transfer means for partially evaporatingthe liquid out of said first fabric tape according to predeterminedprinting information, a second fabric tape, means for transferringprinting ink onto said second fabric tape in an inking station, and atransfer printing point includinga heating device, means for jointlyguiding said first fabric tape and said second fabric tape, one on topof the other, over said heating device, said first fabric tape being intouch contact with said heating device, with a result that liquidcontained in said first fabric tape evaporates, and a pressure elementoperable to press the recording medium relative to said second fabrictape in a direction of said heating device.
 2. The thermal transferprinting device as claimed in claim 1, wherein said first fabric tapecomprises a high-grade steel wire fabric.
 3. The thermal transferprinting device as claimed in claim 1, wherein said second fabric tapecomprises a polyester fabric having a fabric thickness of <100μm.
 4. Thethermal transfer printing device as claimed in claim 1, wherein saidheating device comprises a heatable roller extending transverselyrelative to a direction of transport of the recording medium.
 5. Thethermal transfer printing device as claimed in claim 1, furthercomprising:an ink carrying roller in said inking station; a backing rollmounted to be selectively movable in said inking station to bring saidink-carrying roller into touch contact with said second fabric tape. 6.The thermal transfer printing device as claimed in claim 5, wherein saidink carrying roller is a first ink carrying roller and furthercomprising:a second ink carrying roller in said inking station arrangedin succession in a running direction of said second fabric tape.
 7. Thethermal transfer printing device as claimed in claim 1, wherein saidprinting image transfer means includes a thermal comb having heatingelements activatable independently of one another, one heating elementcorresponding to one pixel of said printing image.
 8. The thermaltransfer printing device as claimed in claim 1, wherein said means forwetting includes a dampening unit which is arranged at said first fabrictape and in which said first fabric tape is wetted with nontoxic liquid.9. The thermal transfer printing device as claimed in claim 8, whereinsaid dampening unit includes two rollers rolling one on the other,between which said first fabric tape can be transported through and oneof which transfers the liquid onto said first fabric tape.
 10. Thethermal transfer printing device as claimed in claim 1, wherein saidfirst and second fabric tapes are endless tapes.